A complex integrated system typically contains a large number of operational amplifiers on a single chip for performing various electronic operations. FETs, such as MOSFETs (Metal-Oxide on Semiconductor Field-Effect Transistors) have certain favorable characteristics for use in such operational amplifiers. They can be fabricated with a minimum of processing steps, they are small, they dissipate very little power, and they present a very high input impedance. One example of an MOS operational amplifier is a CMOS (Complementary MOS) arrangement described by G. M. Jacobs et al. in "TOUCH-TONE Decoder Chip Mates Analog Filters with Digital Logic", Electronics, page 112, FIG. 7, Feb. 15, 1979.
There are, on the other hand, also certain unfavorable characteristics of MOSFETs. For one thing, the electrical characteristics of MOSFETs generally vary considerably from chip to chip. The effects of this can usually be remedied by increasing the complexity of the circuit. A major persisting problem for MOSFET operational amplifiers, however, is that MOSFET devices have a relatively low transconductance. Therefore, when it is required to drive a low impedance load, as is frequently the case, it is often ncessary to provide the operational amplifier with bipolar output transistors in order to increase the output capability. Such bipolar transistors require processing steps additional to those which would be required for an integrated circuit with MOSFET devices alone, and therefore add significantly to the cost of the entire device. Hence there is a need for an operational amplifier which has a large gain, or transconductance, but nevertheless uses MOSFET output transistors.